In recent years there has been increasing concern in many countries about the ease of access to pornographic or violent material, and in particular the effects it may have on young people. Parents and many governments have thus identified a need to enable control of access of certain groups of people to certain types of material. In the past, controls imposed to achieve this aim involved physically preventing unauthorised people accessing restricted recordings: for example, classifying movies according to a minimum age for which they are suitable and then restricting entry to cinemas, and legislating on the minimum age of person to which pornographic magazines may be sold.
As the availability of video material has increased with the ready availability of video tape recordings and domestic equipment on which they can be played, and with the enormous increase of material (including image and video collections) which is available from Internet-connected server computers and the development of video-on-demand services providing remote computer access to video recordings via server computers, the importance of protection and control has increased and the effectiveness of the existing controls has reduced. There is a requirement for controls whereby certain classifications of video and image material cannot be viewed by unauthorised people--such as by enabling parents to control whether a given class of video material will be viewable by their children or not.
A number of solutions to this problem have been suggested which involve transmitting or recording a classification code together with program material for use in automatic censorship or control.
For example, U.S. Pat. No. 4,930,158 discloses recording a program classification code as a digital word located on a specific line of the video signal. This code is recovered when the video recording is played and is used to inhibit replay if the code matches any of a set of user-specified codes. Parents can set the codes when recording a program to prevent children viewing that program.
U.S. Pat. No. 4,930,160 discloses automatic censoring of video programs by receiving a program and a classification code indicative of program content and, for certain prescribed classification codes, switching display to an alternative source so that alternative program material is displayed. The classification code may be encoded into a broadcast or separate from it. A `set classification` routine is used for entering the classifications to be censored as an 8 bit word.
U.S. Pat. No. 5,253,066 discloses use of a signal indicating a class of program being viewed or recorded which signal is additional to a program signal. A viewer selects a set of classifications to be permitted. While a received program indicating signal indicates that the current program is of a permitted classification, recording or viewing of a program is enabled.
U.S. Pat. No. 5,387,942 discloses use of a portion of the blanking interval of a video signal for inserting a digital program code indicating program content. This code is then transmitted along with the audio and video information and is subsequently extracted for use in blocking the display of certain signals if the code matches any of a set of stored codes.
U.S. Pat. No. 5,195,135 discloses frame-by-frame intra-program censorship, using classification data encoded in an audio-video signal. The classification codes classify program material for several different censorship levels. Censorship is then achieved by automatically obscuring the audio and/or video signal in a manner which blurs the image rather than blanking or substituting the image.
EP-A-0549169 discloses a video encoder inserting identification information in the active image portion of a video signal. Tagging data comprising a series of signal pulses causing the video waveform to cross a predetermined threshold is inserted in the active portions of fields at pseudo-random locations, the positions of the pulse marks pseudo-randomly changing position with sequential fields. Such tagging data is somewhat more difficult to detect and remove than identification information which is embedded in a non-displayed portion of the signal (such as in the vertical blanking interval or the synchronisation portion). However, the tagging of EP-A-0549169 relies on the persistence of human vision and the ever-changing position of the inserted marks between each frame for those marks to not be perceptible to the human eye when displayed normally. Nevertheless, the tagging data according to EP-A-0549169 comprises distinct transitions in the video signal which are relatively easy to detect within a given image frame and to remove electronically.
Each of the above prior art disclosures identifies a solution for video program censoring which involves the addition of classification information as a distinct and easily identifiable addition to the image data. Since such classification information would be relatively easy to detect, it would also be relatively easy to remove and a video presentation control system which is dependent on such codes could be by-passed using relatively simple electronics. If censorship controls are to be effective, the classification codes and controls which use those codes must be difficult to tamper with and this remains a problem which has not been adequately solved by the existing solutions.
The US Telecommunications Act of 1996 included initiatives for enabling parental control over what is presented via television screens--effectively requiring inclusion of a viewing controller computer chip or "V-chip" within new television sets. The V-chip has been proposed as a means to enable automatic blocking of presentation of certain television programs on the basis of a content rating system. Broadcasters and other providers of video programming in the US have been encouraged to transmit or record program content information for detection by the V-chip, and television manufacturers will be required to implement the necessary electronics to respond to this information. When installed on a television or a set-top device such as a cable television signal receiver unit, or a satellite communication receiver unit, the V-chip is intended to allow the viewer to customize their family's program reception to prevent display of certain types of program. While the recent legislation has increased the attention given to censorship and television viewing controls, it has not resulted in any significant technical advances in blocking technology. To date, V-chip proposals have involved the use of unallocated bandwidth in the vertical blanking interval of a video signal for the purpose of transmitting program content information.
It has been recognised that similar technology can be implemented within computers for controlling material accessed via the internet. For internet-specific on-line content control, a common protocol for ratings labels known as the Platform for Internet Content Selection (PICS) has been developed. PICS allows attachment of universally recognisable electronic labels to the text or image in documents to alert parents about the nature of their content before the computer displays them or passes them on to another computer. Ratings may be embedded by the publisher of the material, a company providing internet access, or others. To date, the ratings labels used in this area have been `visible` (i.e. easily detectable) labels.
There remains a need for an improved system and method for content-based control of the presentation of video material.